One of the largest problems faced by municipalities, and by society in general, is the increase in the amount of municipal solid waste and other wastes generated each year. For instance, 220 million tons of waste is produced each year in North America alone. A study by the United States Environmental Protection Agency (EPA) has revealed that there has been a 42% increase in recycling and composting from 1988 to 1995. Surprisingly, during this same period, the quantity of municipal solid waste still increased. Certified landfill capacity is decreasing and other sites require clean-up. New options of waste management to replace traditional methods e.g. open dumping, landfills and composting, are needed in order to manage the millions of tons of municipal solid waste that are produced each year.
One way to alleviate the above problems is to convert waste to energy. Most waste-to-energy systems used are so-called through-systems i.e. the waste is combined with fuel as required and burned. In a typical 50 megawatt plant, approximately 5000 tons of waste would be burnt per day. There are, however, several disadvantages to this system. For instance, 50 to 60% of the waste processed is transformed into incompletely burned bottom ash. In addition, fly ash is produced, which is classified as hazardous waste. The bottom ash would have to be shipped to a landfill site and the fly ash would have to be sent to a hazardous waste facility.
The burning of raw municipal solid waste also creates heavy metal emissions, especially from the burning of plastics, aluminum and batteries contained in a general unsorted waste stream. An expensive off-gas treatment installation is necessary to reduce these emissions to levels required to meet environmental and other regulations. In addition, if the municipal solid waste is wet, it would, in general, cost more in energy to burn the waste than the actual amount of energy produced from the waste. Nonetheless, production of waste-derived fuel has evolved to become an acceptable approach to waste management. For instance, the waste may be stripped of metal and glass components, and then granulated, shredded and subsequently dried to yield a product with a higher fuel value and lower ash production than an incineration process.
U.S. Pat. Nos. 3,473,494 and 3,584,587 disclose a system for processing raw municipal solid waste, in which non-ferrous waste is passed through a grinder, a shredder and a dryer. Incineration of the resultant product in a furnace results in a by-product of slag or ash, mainly consisting of glass and some plastics, in the form of solid clinker in the base of the furnace. Periodic shut-downs are required for removal of such clinkers. Canadian Patent No. 1,038,335 discloses a further processing step that passes the shredded and dried non-ferrous waste through another separation step in which heavier shredded particles of non-ferrous metal, glass and plastic are separated from lighter particles such as paper, vegetation, and pulp. The lighter particles, in the form of a fluff, are screened to remove dust or grit and then incinerated.
EP 0 036 784 discloses pulverizing municipal solid waste, and then screening to remove fines and oversize material. A light fraction, containing mainly paper and plastics, is separated from the screened residue and pelletized. The ideal moisture content is 10% to 20% by weight, which is lower than a typical wet pulping process e.g. as disclosed in British Patent No. 1,517,687. In the latter, a combustible fraction of waste is obtained from an aqueous slurry containing about 4% to 6% solid waste, by extraction of organic components. The combustible fraction is dried to a moisture content of 10 to 20% by weight, and then pelletized. The energy consumed in the drying step has to be balanced against the energy available from the pelletized fuel. The high water content and the foul aqueous medium obtained detract from the wet pulping process.
British Patent No. 2,024,856 discloses a process wherein a light fraction extracted from municipal solid waste and selectively containing plastics, wood, rags and paper, is subjected to a milling process. The moisture content of the resulting waste may be controlled.
European Patent No. 566,419 discloses introducing an additive, such as calcium oxide/calcium-based bentonite, to comminuted municipal solid waste for odour control.
U.S. Pat. No. 4,496,365 discloses a method of producing fuel briquettes from organic waste products enriched with organic materials such as coal, by adding lime and subsequently compressing the mixture under high pressure. In North America, governments are seeking to phase out coal-fired facilities because of the emissions, such as sulphur, that are produced.
U.S. Pat. No. 5,888,256 discloses a method of producing a fuel by blending one or more waste materials selected from paper mill waste, paper mill lime waste, municipal waste water treatment sludge, wood waste, paper waste, plastic-containing manufacturing waste, fly ash, petroleum-containing sludge, organic agricultural wastes, rubber-containing waste and inorganic mineral-containing waste. A pellet of at least 5500 BTU/lb, or, more preferably, 7000 to 9000 BTU/lb is obtained. It is to be expected that the pellet could cause heavy metal emissions.
Biogas systems are also known. Methane gas can be produced from the fermentation of waste materials such as municipal solid waste, organic waste and food matter. The resultant product is approximately 98% of the input waste, and may be sent to landfill. However, the product is not suitable for composting because the organic and food matter that makes good compost has been used in the methane gas production process. Compost of poor quality is obtained.
Many of the aforementioned processes involve processing the raw municipal solid waste such that a light fraction more suitable for combustion is separated and burned. Therefore, the entire waste is not completely utilized. The heavier fraction of waste must still be disposed of or utilized e.g. as earth fill. It would be more effective if the raw municipal solid waste could be utilized in a more efficient and less hazardous manner.
A process is required that will utilize the majority of municipal solid waste, thereby eliminating or reducing the need for landfill sites and composting. In addition, the waste should provide a high fuel value for generation of energy.